Catalytically active particulate filter and use thereof

ABSTRACT

A catalytically active particulate filter is proposed which is suitable for use in an exhaust gas cleaning system for diesel engines. The particulate filter removes diesel soot particles from the exhaust gas and is also effective to oxidize carbon monoxide and hydrocarbons and to convert nitrogen monoxide at least proportionally into nitrogen dioxide. The particulate filter comprises a filter body ( 3 ) and two catalytically active coatings ( 1 ) and ( 2 ) which contain platinum and palladium, or platinum or palladium respectively, wherein the platinum content of the second catalytically active coating ( 2 ) is higher than the platinum content of the first catalytically active coating ( 1 ).

The invention concerns a catalytically active particulate filter and usethereof to clean the exhaust gases of diesel engines, in particular aspart of an exhaust gas cleaning system containing a diesel oxidationcatalyst, a diesel particulate filter and a catalyst for selectivecatalytic reduction of nitrous oxides, which are connected successivelyin this order in the flow direction of the exhaust gas.

The untreated exhaust gas from diesel engines contains, as well ascarbon monoxide CO, hydrocarbons HC and nitrous oxides NO_(x), arelatively high oxygen content of up to 15 vol. %. It also containsparticulate emissions which consist primarily of soot residues and whereapplicable organic agglomerates, and arise from a partially incompletefuel combustion in the cylinders.

The observation of exhaust limit values for diesel vehicles imposed byfuture laws in Europe, North America and Japan requires the simultaneousremoval of particulates and nitrous oxides from the exhaust gas. Thepollutant gases carbon monoxide and hydrocarbon from the lean exhaustgas can be rendered harmless by oxidation on a suitable oxidationcatalyst. To remove the particulate emissions, diesel particulatefilters with and without additional, catalytically active coating aresuitable. The reduction of nitrous oxides into nitrogen(“denitrification” of the exhaust gas) is more difficult because of thehigh oxygen content. A known method is selective catalytic reduction(SCR) of the nitrous oxides on a suitable catalyst known as an SCRcatalyst. This method is presently preferred for denitrification ofdiesel engine exhaust gases. In the SCR process, the nitrous oxidescontained in the exhaust gas are reduced by means of a reducing agentintroduced into the exhaust gas line from an external source. Apreferred reducing agent is ammonium, wherein in particular compoundsreleasing ammonium such as urea or ammonium carbamate are added. Theammonium where applicable generated in situ in this manner from theprecursor compound reacts at the SCR catalyst with the nitrous oxidesfrom the exhaust gas into nitrogen and water in a corn proportioningreaction.

At the present time, a combination of different exhaust gas cleaningdevices is unavoidable in order to meet the future legal requirements. Adevice for cleaning diesel engine exhaust gases must contain at leastone oxidation-active catalyst, and for denitrification an SCR catalystwith an upstream device for introducing the reducing agent (preferablyammonium or urea solution) and an external reducing agent source (forexample an additional tank with urea solution or an ammonium store). Ifoptimisation of the engine combustion process does not succeed inkeeping particulate emissions so low that these can be removed via theoxidation catalyst by direct oxidation with oxygen, in addition the useof a particulate filter is required. Here the use of special,catalytically activated particulate filters is possible. Such aparticulate filter is disclosed for example in EP 1 486 248 A1. Thediesel particulate filter has an inlet side for the inflowing exhaustgas and an outlet side for outflowing exhaust gas. It contains asubstrate with an inner wall surface and an outer wall surface. Acatalytically active coating is applied on the inner wall surface of theinlet side, while a second catalytically active coating is applied onthe outer wall surface towards the outlet side. The catalytically activecoatings can have various compositions and different catalyticfunctions.

Exhaust gas cleaning systems which are constructed from several exhaustgas cleaning devices connected in series are well known from the priorart; some are presently undergoing practical trials or are in seriesintroduction. For example EP 1 054 722 A1 describes a system fortreating diesel exhaust gases containing NO_(x) and particulates,wherein an oxidation catalyst is connected upstream of a particulatefilter. On the downstream side of the particulate filter are connected areducing agent source and a metering device for the reducing agent, andan SCR catalyst. In the manner described here, the NO₂ proportion in theexhaust gas and hence the NO₂/NO_(x) ratio is increased by at leastpartial oxidation of NO on the oxidation catalyst, wherein the NO/NO₂ratio is preferably “set to a predefined level optimum for the SCRcatalyst”.

This NO/NO₂ ratio optimum for the SCR catalyst lies in the region of 1for all SCR catalysts known at present. If the NO_(x) contained in theexhaust gas consists solely of NO and NO₂, the optimum NO₂/NO_(x) ratiois between 0.3 and 0.7, preferably between 0.4 and 0.6, and particularlypreferably 0.5. Whether this ratio is achieved before the SCR catalystin a system according to EP 1 054 722 A1 depends on the exhaust gastemperature and hence on the operating state of the engine, the activityof the oxidation catalyst, the precious metal charge, and the structureand soot charge of the diesel particulate filter connected downstream ofthe oxidation catalyst. The fact that the NO/NO₂ ratio in the exhaustgas is to be set via the oxidation catalyst, wherein the thus configuredexhaust gas must still flow through the downstream diesel particulatefilter before reaching the SCR catalyst, means that in dynamic normaloperation of the vehicle it cannot always be ensured that this NO/NO₂ratio in the exhaust gas after the particulate filter and before the SCRcatalyst always lies in the range optimum for the SCR reaction. Theresult is NO_(x) breakthrough due to incomplete reduction and/orammonium breakthrough due to ammonium overdose.

WO 2009/140989 solves this problem by the use of a catalyticallyactivated particulate filter in a system according to EP 1 054 722 A1with simultaneous adaptation of the catalytic coating of the upstreamoxidation catalyst. In a device for cleaning diesel exhaust gases whichin the flow direction of the exhaust gas comprises an oxidationcatalyst, a diesel particulate filter with catalytically active coating,a device for introducing a reducing agent from an external reducingagent source, and an SCR catalyst, both the oxidation catalyst and thecatalytically active coating of the diesel particulate filter containpalladium and platinum. The ratio of the total quantity of palladium tothe total quantity of platinum in the oxidation catalyst and particulatefilter is between 8:1 and 1:15. Also the ratio of platinum to palladiumin the oxidation catalyst is no greater than 6:1, while thecatalytically active coating of the diesel particulate filter isselected such that the ratio of platinum to palladium present therein isno less than 10:1.

It is evident from present developments of diesel engines andapplications that in future diesel engines, the exhaust gas to becleaned—when it leaves the engine and enters the exhaust gas cleaningsystem—is becoming ever colder. The avoidance of additional heat lossesvia the exhaust line requires an ever more compact construction of theexhaust gas cleaning system. Consequently the installation volumeavailable for the upstream oxidation catalyst is becoming ever smaller.The smaller dimensioning of the oxidation catalysts, in conjunction withthe reduction in normal operating temperature, leads to the consequencethat the hydrocarbons and carbon monoxide contained in the exhaust gasesto be cleaned often can no longer be fully converted into carbon dioxideat the oxidation catalyst. Also under these conditions the oxidationcatalyst is unable to set a NO/NO₂ ratio which is even onlyapproximately optimum for denitrification of the exhaust gas in the SCRcatalyst located at the end of the exhaust line. Consequently theresidual emissions of carbon monoxide and in particular hydrocarbons,but also particularly environmentally critical nitrous oxides, remainingin the exhaust gas are too high to meet the future applicable exhaustgas limit values.

The object of the present invention is to provide an exhaust gascleaning device for cleaning the exhaust gases from diesel engines whichis suitable for use in an exhaust gas cleaning system with dieseloxidation catalyst, diesel particulate filter and SCR catalyst, withoutthe above-mentioned disadvantages of conventional systems.

This object is achieved by a catalytically active particulate filterwhich removes soot particles from the exhaust gas and is able to oxidisecarbon monoxide and hydrocarbon and to convert nitrogen monoxides atleast proportionally into nitrogen dioxide.

The object of the present invention is a catalytically activeparticulate filter comprising

-   -   as a filter body, a wall flow filter substrate (3) which        comprises inflow and outflow channels separated by porous walls        (7), wherein the outflow ends of the inflow channels (4) and the        inflow ends of the outflow channels (5) are sealed gas-tight        (6), and    -   at least two catalytically active coatings, wherein the first        catalytically active coating (1) contains platinum and palladium        and is situated in the porous walls (7) between the inflow and        outflow channels, and the second catalytically active coating        (2) contains platinum or palladium and platinum and is situated        in the outflow channels on the porous walls (7) between the        inflow and outflow channels,        wherein the platinum content of the second catalytically active        coating (2) is higher than the platinum content of the first        catalytically active coating (1).

The object of the present invention is furthermore a method for reducingparticulates, hydrocarbons and carbon monoxide in diesel engine exhaustgases, wherein the exhaust gas to be cleaned containing carbon monoxide,hydrocarbons, particulates and nitrous oxides, including nitrogenmonoxide, is passed through a catalytically active particulate filteraccording to the invention.

The catalytically active particulate filter according to the inventionis optimised for use in an exhaust gas system which for this on theinflow side has a diesel oxidation catalyst and on the outflow side adevice for addition of ammonium or a compound releasing ammonium, and anSCR catalyst to convert nitrous oxides with ammonium into nitrogen.

The object of the present invention is therefore also an exhaust gascleaning system which in the flow direction of the exhaust gas comprisesan oxidation catalyst, a diesel particulate filter with catalyticallyactive coating, a device for introducing a reducing agent from anexternal reducing agent source, and an SCR catalyst, which ischaracterized in that it contains a catalytically active particulatefilter according to the invention as a diesel particulate filter withcatalytically active coating.

Preferably the first catalytically active coating (1) has a Pt:Pd weightratio of 1:4 to 2:1.

In a particular embodiment of the catalytically active particulatefilter according to the invention, the first catalytically activecoating contains one or more zeolite compounds which have a bufferingeffect against the hydrocarbons occurring in the diesel exhaust gas.These are preferably selected from the group of beta-zeolite, X-zeolite,Y-zeolite, mordenite and ZSM-5 zeolite.

Said zeolite compounds are preferably used in quantities of 0.5 to 20g/l by volume of the wall flow filter substrate.

Preferably the first catalytically active coating (1) extends over theentire length of the wall flow filter substrate.

The composition of the first catalytically active coating is selectedsuch that at moderate exhaust gas temperatures, it catalyses theoxidation of hydrocarbons and carbon monoxide into carbon dioxide ascompletely as possible. This guarantees that carbon monoxide andhydrocarbons which “break through” any upstream diesel oxidationcatalyst are oxidised into carbon dioxide as fully as possible at thelatest at this point in the exhaust gas cleaning system and consequentlyare not emitted to the environment.

The second catalytically active coating (2) has a higher platinumcontent than the first catalytically active coating (1). Preferably theplatinum content of the second catalytically active coating (2) is 1.2to 3 times higher than the platinum content of the first catalyticallyactive coating (1).

Preferably the second catalytically active coating (2) contains moreplatinum than palladium. Particularly preferably the Pt:Pd weight ratiois more than 6:1, particularly preferably 12:1. In embodiments in whichthe second catalytically active coating (2) requires a particularly highoxidation power against nitrogen monoxide, it may contain only platinumand be free from palladium (weight ratio 1:0).

Preferably the second catalytically active coating (2) extends over theentire length of the wall flow filter substrate.

The second catalytically active coating (2) is composed such that it isable to oxidise at least part of the nitrogen monoxide contained in theexhaust gas into nitrogen dioxide in order thus in total to provide agreater quantity of NO₂ to the downstream SCR catalyst. This ensuresthat after passing through the catalytically active particulate filteraccording to the invention, the exhaust gas—depending on exhaust gastemperature and precious metal charge—has an NO₂/NO_(x) ratio of 0.3 to0.7, preferably 0.4 to 0.6, and particularly preferably 0.5.

The platinum and palladium, or platinum, contained in the twocatalytically active coatings are usually present on an oxidic carriermaterial selected from the group consisting of aluminium oxide, silicondioxide, rare earth oxide, titanium oxide and zirconium oxide, or mixedoxides, or mixtures thereof. A preferred oxidic carrier material isaluminium oxide which in particular is stabilised with lanthanum oxide.In the latter case lanthanum oxide is present preferably in quantitiesof 2 to 6 w. % in relation to the stabilised aluminium oxide.

The physical separation of the two catalytically active coatingsaccording to the present invention surprisingly causes an overallimproved conversion of all pollutants. In particular it leads to thefact that the NO conversion into NO₂ observed over the completecomponent is significantly higher than in conventional components inwhich the catalytically active constituents are present in a singlecoating.

The wall flow filter substrate preferably consists of a ceramic materialsuch as for example cordierite, silicon carbide or aluminium titanate.

FIG. 1 shows the catalytically active particulate filter according tothe invention in diagrammatic form. The first catalytically activecoating (1) is arranged on the inflow channel side in the porous wallsbetween the inflow channels (4) and outflow channels (5). The secondcatalytically active coating (2) is arranged on the outflow channel sideon the walls between the inflow channels (4) and the outflow channels(5).

The soot particles cannot penetrate the porous walls and are completelyfiltered out of the exhaust gas on the inflow side. This has theadvantage that the second catalytically active coating which is used forNO oxidation remains soot-free.

The catalytically active particulate filter according to the inventionis suitable in particular for use in a method for reduction ofparticulates, hydrocarbons and carbon monoxide, wherein the exhaust gasto be cleaned contains, as well as the above-mentioned components, alsonitrous oxides including nitrogen monoxide. The first catalyticallyactive coating (1) is effective to oxidise hydrocarbons and carbonmonoxide, while the second catalytically active coating (2) is effectiveto convert at least part of the nitrogen monoxide present in the exhaustgas into nitrogen dioxide. According to the method in the invention, notonly are the hydrocarbons, carbon monoxide and particulates reduced inthe exhaust gas to be cleaned. Also the NO₂/NO_(x) ratio in the exhaustgas is set to a value of 0.3 to 0.7. Thus the exhaust gas is prepared inthe optimum manner for the removal of nitrous oxides by comproportioningwith ammonium at a downstream SCR catalyst.

For subsequent denitrification of the exhaust gas, downstream of thecatalytically active particulate filter according to the invention,preferably in this order, are connected (a) a device for meteredaddition of ammonia or a compound releasing ammonium, and (b) an SCRcatalyst which is effective to convert nitrous oxides with ammonium intonitrogen. The total cleaning effect of the method is further increasedif furthermore upstream of the catalytically active particulate filteraccording to the invention is connected an oxidation catalyst which iseffective to oxidise carbon monoxide and hydrocarbons.

As a whole, a highly effective exhaust gas cleaning system is achievedfor reducing hydrocarbons, carbon monoxide, particulates and nitrousoxides from the exhaust gases of diesel engines, which featuressubstantially improved cleaning efficiency in comparison with systemsaccording to the prior art. In particular the system according to theinvention is suitable for cleaning the exhaust gases of more moderndiesel engines, the exhaust gases of which have comparatively lowtemperatures.

The invention is now explained in more detail below with reference tosome examples and figures. These show:

FIG. 1: a diagrammatic depiction of a catalytically active particulatefilter according to the invention comprising:

-   -   (1) a first catalytically active coating which is effective to        oxidise hydrocarbons and carbon monoxide;    -   (2) a second catalytically active coating which is effective to        convert at least part of the nitrogen monoxide present in the        exhaust gas into nitrogen dioxide;    -   (3) as a filter body, a wall flow filter substrate comprising:    -   (4) inflow channels through which exhaust gas flows into the        substrate;    -   (5) outflow channels through which the exhaust gas flows out of        the substrate; wherein    -   (6) both inflow channels and outflow channels are each closed        gas-tight on one side.

EXAMPLE

For production of a particulate filter according to the invention, awall flow filter substrate of silicon carbide was fitted with coatings(1) and (2) as indicated diagrammatically in FIG. 1. The substrate had adiameter of 143.8 millimeters and a length of 152.4 millimeters. It hada cell density of 300 cells per square centimeter with a wall thicknessof 0.33 millimeters. The total cell charge was 35.4 g/ft³ with a Pt/Pdratio of 6.1:1.

a) First a coating suspension was created to produce the coating (1)suitable for oxidation of hydrocarbons and carbon dioxide. For this analuminium oxide stabilised with lanthanum oxide (4 w. % La₂O₃ inrelation to the total mass of the mixed oxide, BET surface approx. 180m²/g) was moistened with a water-based solution of tetra-amine platinumacetate and tetra-amine palladium nitrate, filling the pores, whereinthe pourability of the powder was retained.

The precious metal content of the solution and the precious metal ratiowere selected according to the target quantity to be achieved and theprecious metal ratio to be achieved in the coating (1). The targetcharge was 15 g/ft³ with a Pt/Pd ratio of 2.1:1.

To fix the precious metal, the damp powder was calcinated for 4 hours at300° C.

The resulting catalytically activated powder was suspended in water,provided with a zeolite and ground after setting the pH until it had aparticle size distribution with a d₁₀₀ value of less than 7 micrometers.The coating of the wall flow filter substrate was applied partly bymachine. For this the filter substrate was oriented vertically in thecoating chamber of a coating machine according to the prior art so thatthe inlet face of the later inflow side of the component faceddownwards. Then the coating suspension (1) was pumped into the substratefrom below until the inflow channels of the substrate were filled withsuspension over their entire length. Then the coating suspension (1) waspumped out again from below and then extracted, wherein the pump-out orextraction power was selected such that the proportion of solidscontained in the suspension remained in the channel walls of the wallflow filter substrate in the desired quantity.

A coating quantity to be applied of 15 to 20 grammes per liter inrelation to the substrate volume was selected and introduced into thewall of the wall flow filter substrate.

b) For production of the coating (2) suitable for at least partialoxidation of NO, the procedure was similar to the process describedabove. The washcoat provided for the coating (2) was ground to aparticle size of 8-10 micrometers (d₁₀₀) and applied to the wall of theoutlet channel by an immersion coating process from the outlet end ofthe wall flow filter substrate. The target charge of the coating (2) was20.4 g/ft³ with a Pt/Pd ratio of 1:0.

The resulting catalytically activated diesel particulate filter wascalcinated for a period of 4 hours at 300° C. and then treated withforming gas for 2 hours at 500° C.

The resulting catalytically active particulate filter was subjected to asynthetic ageing procedure before characterisation. For this the filterbodies were exposed to an atmosphere of 10 vol. % water vapour and 10vol. % oxygen in nitrogen for a duration of 16 hours in an oven at 800°C.

Comparison Examples 1 and 2

For comparison, two wall filter substrates were coated conventionally,i.e. the entire charge was introduced homogeneously into the channelwalls of the substrate. The coating thus corresponded to coating (1) ofthe particulate filter according to the invention in the example butcontained the entire quantity of precious metal.

First coating suspensions were produced. For this an aluminium oxidestabilised with lanthanum oxide (4 w. % La₂O₃ in relation to the totalmass of the mixed oxide, BET surface approx. 180 m²/g) was moistenedwith a water-based solution of tetra-amine platinum acetate andtetra-amine palladium nitrate, filling the pores, wherein thepourability of the powder was retained. The precious metal content ofthe solution and the precious metal ratio were selected according to thetarget quantities and ratios to be achieved. The target charge ofcomparison example 1 was 34 g/ft³ with a Pt/Pd ratio of 12:1 (costequivalence), and of comparison example 2, 40 g/ft³ with a Pt/Pd ratioof 2:1.

To fix the precious metal, the damp powder was calcinated for 4 hours at300° C.

The resulting catalytically activated powder was suspended in water,provided with a zeolite and ground after setting the pH until it had aparticle size distribution with a d₁₀₀ value of less than 7 micrometers.

The coating of the wall flow filter substrate was applied partly bymachine. For this the filter substrate was oriented vertically in thecoating chamber of a coating machine according to the prior art so thatthe inlet face of the later inflow side of the component faceddownwards. Then the coating suspension concerned was pumped into thesubstrate from below until the inflow channels of the substrate werefilled with suspension over their entire length. Then the coatingsuspension was pumped out again from below and then extracted, whereinthe pump-out or extraction power was selected such that the proportionof solids contained in the suspension remained in the channel walls ofthe wall flow filter substrate in the desired quantity.

A coating quantity to be applied of 15 to 20 grammes per liter inrelation to the substrate volume was selected and introduced into thewall of the DPF.

The comparison specimens had a charge of 34 g/ft³ and 40 g/ft³ with aprecious metal ratio of Pt/Pd of 12:1 and 2:1. These charges arecost-equivalent to the catalytically active particulate filter describedin example 1.

The resulting comparison filters were hydrothermally aged in the sameway as described for example 1.

Comparison Experiment

After the hydrothermal ageing described above, the catalytically activeparticulate filter according to the invention according to example 1 andthe comparison filters according to the comparison examples wereconnected in succession in a DPF canning to an exhaust gas line of a 2.0l diesel engine of standard Euro 4 and exposed to approximately 8 g/lsoot. After the end of the sooting process, the temperature was rampedup to 300° C. and the so-called light-off of the particulate filter andNO₂ formation measured.

For the same HO/CO performance, the catalytically active particulatefilter according to the invention had a 30% (relative) higher NO₂formation after sooting than the conventionally coated comparisonfilter. The catalytically active particulate filter according to theinvention therefore provides a substantially better performance for thesame precious metal costs.

The invention claimed is:
 1. Catalytically active particulate filter comprising as a filter body, a wall flow filter substrate which comprises inflow and outflow channels separated by porous walls, wherein the outflow ends of the inflow channels and the inflow ends of the outflow channels are sealed gas-tight, and at least two catalytically active coatings, wherein the first catalytically active coating contains platinum and palladium and is situated in the porous walls between the inflow and outflow channels, and the second catalytically active coating contains platinum or palladium and platinum and is situated in the outflow channels on the porous walls between the inflow and outflow channels, wherein the platinum content of the second catalytically active coating is higher than the platinum content of the first catalytically active coating.
 2. Catalytically active particulate filter according to claim 1, wherein the first catalytically active coating has a Pt:Pd weight ratio of 1:4 to 2:1.
 3. Catalytically active particulate filter according to claim 1 wherein the first catalytically active coating contains one or more zeolite compounds.
 4. Catalytically active particulate filter according to claim 1, wherein the first catalytically active coating extends over the entire length of the wall flow filter substrate.
 5. Catalytically active particulate filter according to claim 1, wherein the second catalytically active coating contains more platinum than palladium.
 6. Catalytically active particulate filter according to claim 1, wherein the second catalytically active coating has a Pt:Pd weight ratio of more than 6:1.
 7. Catalytically active particulate filter according to claim 1, wherein the second catalytically active coating extends over the entire length of the wall flow filter substrate.
 8. Catalytically active particulate filter according to claim 1, wherein a wall flow filter substrate of ceramic material is used as the filter body.
 9. A method for reduction of particulates, hydrocarbons and carbon monoxide in diesel engine exhaust gas, comprising; passing the exhaust gas to be cleaned containing carbon monoxide, hydrocarbons, particulates and nitrous oxides, including nitrogen monoxide, through a catalytically active particulate filter according to claim
 1. 10. Method according to claim 9, wherein after passing through the catalytically active particulate filter, the exhaust gas has a NO₂/NO_(x) ratio of 0.3 to 0.7.
 11. Method according to claim 9, wherein downstream of the catalytically active particulate filter, in this order, the exhaust gas is subjected to (a) a device for metered addition of ammonium or a compound releasing ammonium, and (b) an SCR catalyst which is effective to convert nitrous oxides with ammonium into nitrogen.
 12. Method according to claim 9, wherein upstream of the catalytically active particulate filter is connected an oxidation catalyst which is effective to oxidise carbon monoxide and hydrocarbons in the exhaust gas.
 13. An exhaust gas cleaning system which in the flow direction of the exhaust gas comprises an oxidation catalyst, a diesel particulate filter, a device for introducing a reducing agent from an external reducing agent source, and an SCR catalyst, wherein the diesel particulate filter is a catalytically active particulate filter according to claim
 1. 